This invention provides a new and improved system and method of controlling resonant free piston Stirling engines (RFPSE) operated as a "thermal amplifier." U.S. Pat. No. 4,434,617, filed, July 27, 1982 and issued Mar. 6, 1984entitled, "Start-up and Control Method and Apparatus for Resonant Free Piston Stirling Engine"--Michael M. Walsh, inventor and assigned to Mechanical Technology Incorporated, Latham, N.Y. discloses an RFPSE system and method operated as a "thermal oscillator." In U.S. Pat. No. 4,434,617, the Stirling engine is designed for operation as a free running "thermal oscillator" or as a damped "thermal oscillator" by causing a displacer drive motor to be operated as a load.
The RFPSE described in U.S. Pat. No. 4,434,617 has a displacer reciprocally arranged within the housing of the Stirling engine which is subjected to a periodic pressure wave produced in the engine working gas operative to drive a working member or piston from which work is derived from the engine. Also, a linear dynamoelectric machine is arranged and constructed to be in driving arrangement with the displacer of the Stirling engine. A control excitation circuit provides a means for selectively causing the dynamoelectric machine to operate either as a drive motor, to supply drive energy to the displacer member, or to operate as a generator, to apply a load on the displacer. The term generator is used herein in its most generic sense to designate a machine which converts mechanical energy into electrical energy.
While the foregoing briefly described basic arrangement disclosed in U.S. Pat. No. 4,434,617, is entirely satisfactory, there is a continuing need to provide new and improved RFPSE systems and methods of operation thereof, and control systems therefor which more precisely and stably control the Stirling engine drive systems in a more comprehensive and reliable manner, particularly during transients caused by large and rapid load changes.
U.S. Pat. No. 4,215,548 issued Aug. 5, 1980, to Donald G. Beremand for a "Free-Piston Regenerative Hot Gas Hydraulic Engine," which is identified and distinguished over in U.S. Pat. No. 4,434,617, discloses a free piston Stirling engine having a free displacer with means for imparting motion to the displacer (pneumatic, electric or hydraulic). This means alledgedly controls the displacer operation so that the power piston completes its stroke prior to the reversal of the displacer piston and also provides control of the frequency at which the displacer piston is activated. As disclosed in FIG. 4 of that patent, an on-off type electric solenoid provides a convenient means of achieving such operation since by changing the polarity of the solenoid it can be made to drive the displacer to one end of the stroke or the other as required to generate the PV diagram shown in FIG. 6 of the patent. In the Beremand engine, frequency of switching the solenoid on and off controls the engine speed and hence the engine power. Changing frequency is not intended to change the displacer phase angle or stroke since it is stated that the intention of the Beremand system is to maintain the thermodynamic operation of each individual cycle of the engine constant from zero to maximum speed.
In the present invention, in contrast to the Beremand engine, steady state power is varied specifically by changing the thermodynamic operation of the RFPSE during each individual cycle. In particular, both the displacer stroke amplitude and the displacer phase angle are controllably varied by changing the voltage and frequency at which the electrodynamic drive motor is excited. In order for this system to operate properly according to the invention, it is not necessary that the reversal in direction of the displacer occur simultaneous to completion of the power piston stroke. This operation is in direct contrast to the Beremand engine mode of operation.
In the present invention means are provided to precisely control in a variable and stable manner the transient engine operations during rapid load changes in contrast, for example, to the prior Beremand engine and the Walsh engine.